1. Field of the Invention
The present invention relates generally to an image sensor and a method of manufacturing the same. More particularly, the invention relates to an image sensor having enhanced electrical characteristics, and a method of manufacturing the same.
A claim of priority is made to Korean Patent Application No. 2004-76547 filed on Sep. 23, 2004, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
Image sensors are included in a wide array of modern electronic devices. For example, image sensors can be found in digital cameras, personal digital assistants (PDAs), and mobile phones, to name but a few.
The two most popular types of image sensors used in contemporary electronic devices are charge coupled device (CCD) image sensors, and complementary metal oxide semiconductor (CMOS) image sensors. Both types of image sensors convert optical signals into electrical charges and then convert the electrical charges into voltages representing an image. However, there are differences in the way these functions are performed.
In CCD image sensors, the optical signals are converted into electrical charges on one chip, and the electrical charges are converted into voltages by elements (e.g., amplifiers, buffers, drivers, etc.) on a printed circuit board attached to the chip. In contrast, in CMOS image sensors, the optical signals are converted into electrical charges and the electrical charges are converted into voltages all on a single chip.
A pixel sensor (i.e., an apparatus used to generate a single pixel of an image) in a CMOS image sensor typically comprises a photodiode generating electrical charges in response to incident light and a CMOS logic circuit converting the electrical charges into voltages representing an image (i.e., pixel voltages).
Unfortunately, the photodiode tends to generate additional electrical charges due to heat. In order to minimize image noise caused by heat-generated electrical charges, the heat-generated electrical charges ought to be excluded from the pixel voltages. One way to eliminate heat-generated electrical charges from the pixel voltages is to include pixel sensors in the CMOS image sensor with a light shielding layer covering their photodiodes. An area of the CMOS image sensor having pixel sensors covered by the light shielding layer is called an “optical black area” (OBA) and an area of the CMOS image sensor having pixel sensors not covered by the light shielding layer is called an “optical acceptance area” (OAA).
Pixel sensors in the optical black area generate electrical charges by heat but not light. Accordingly, a voltage generated by these pixel sensors serves as a “reference signal(s)” for approximating the amount of heat-generated electrical charges in pixel sensors of the optical acceptance area. The reference signal(s) generated by pixel sensors in the optical black area of the CMOS image sensor can be subtracted from pixel voltages in order to exclude any contribution made by electrical charges generated by heat alone.
Where the light-shielding layer is thin or partially removed from the optical black area, light may be partially irradiated onto photodiodes contained therein. As a result of this irradiated light, charges may be generated in pixel sensors of the optical black area. In general, this increases the amount of electrical charges generated by the pixel sensors in the optical black area, making the resulting reference signal(s) a poor indication of the amount of electrons generated by heat. In other words, it makes the reference signal noisy. Of course, using a noisy reference signal(s) to generate pixel voltages tends to lower the quality of images resulting from the pixel voltages.
Copper (Cu) is generally used to form wiring and light-shielding layers for image sensors because copper has lower electrical resistance than aluminum (Al) or tungsten (W). Copper wiring and light shielding layers for image sensors are typically formed by a damascene process. Unfortunately, the damascene process often causes dishing to occur in the light-shielding layer. The term “dishing” denotes a thinning or non-uniform formation of a portion or portions of the light-shielding layer.
FIG. 1 is a cross-sectional view of a conventional image sensor having a copper light-shielding layer in which dishing occurs.
Referring to FIG. 1, a copper light-shielding layer 10 has a thin portion 12 in an optical black area “I” of a CMOS image sensor but not in an optical acceptance area “II”. Thin portion 12 is caused by a damascene process used to form light-shielding layer 10. Where light-shielding layer 10 has thin portion 12, light may pass through thin portion 12 of light-shielding layer 10, causing reference signals generated by the image sensor to be noisy. As a result, the quality of images produced by the image sensor may be compromised.